1. Field of the Invention
The field of the present invention is nuclear or fossil fuel steam turbines. More particularly, this invention relates to a system for minimizing thermal distortion of low pressure steam turbine stationary blade rings.
2. Description of the Prior Art
Steam turbines have established a wide usefulness as prime movers, and they are manufactured in many different forms and arrangements. Most steam turbines in use today consist of multiple stages, typically 4-12 stages. The turbine stage consists of a stationary set of blades, often called nozzles, and a moving set adjacent thereto, called buckets, or rotor blades. These stationary and rotating blades (typically 60-140 per ring) act together to allow the steam flow to do work on the rotor, which can be transmitted to the load through the shaft on which the rotor assembly is carried.
Despite their many advantages, there are a number of items that lead to inefficiencies in steam turbines. These include friction losses in both the stationary blades and the rotor blades, rotation loss of the rotor, leakage loss between the inner circumference of the stationary blade and the rotor and between the tip of the rotor blades and the casing, and moisture and super-saturation losses if the steam is wet. Another problem encountered in these types of turbines is that of thermal deformation. Most steam turbines are constructed with at least a single upper casing and lower casing, each having radially inwardly extending ribs and including halves of the stationary blade rings and a series of stationary blades. (Others provide a double casing system.) When the two halves of these casings are formed around the central rotor with its rotating blades, they are typically joined together using a horizontal joint flange. Generally, however, the stationary rings are not bolted together, relying instead upon their attachment to the casing and upon the use of steam sealing keys-strips of a metal, such as steel, used to close the gap between blade ring halves.
The horizontal joint flange used to connect the casing halves is a major source of non-axisymmetric thermal deformation during normal operation of the turbine. When subjected to normal hot operating conditions, the hot inlet region of the turbine expands outwardly in all directions against the casing and the casing against the horizontal joint flange. The horizontal joint flange, being the major point of discontinuity along the casing surface, deforms differently than the casing itself and, being joined to the upper and lower portions of the casing, pulls outwardly on the casing ends and pushes inwardly on the center region.
The variety of thermal distress factors lead to problems of fatigue-cracking and bolt breaking in addition to deformation inefficiencies. Because of these problems, it is desirable to isolate, as far as possible, the stationary blade rings from the inner casing. This is particularly true when there is a single inner casing since, in such a case, the blade rings are less isolated from damaging heat gradients than if there were a double casing.
Accordingly, there exists a need for a method or apparatus that can minimize the thermal distortion of stationary blade rings within steam turbines.